RU2727233C2 - Laser device for selective treatment of acne with decreased skin temperature increase - Google Patents

Abstract

FIELD: medical equipment.SUBSTANCE: laser device for selective treatment of acne comprises a laser source terminating in an optical collimator, which supplies a laser beam, wherein said laser source comprises a switch, which makes it possible to transmit pulses of said laser beam of given duration, optomechanical interface containing a lens, a focusing laser beam received from the optical collimator, optical fiber connected to said optomechanical interface. Said optical fiber has a length greater than 15 m, and said device comprises a tip connected to said optical fiber, said tip having an optical zoom system which makes it possible to change the diameter of the laser beam coming out of said tip from 0.5 to 5 mm.EFFECT: disclosed is a laser device for selective treatment of acne with reduced skin temperature increase.9 cl, 9 dwg

Description

The present invention relates to a laser device for the selective treatment of acne with a reduced rise in skin temperature.

A compact hand-held portable device for the treatment of sebaceous gland disorders in the dermal region of the skin is described in document WO2008 / 008971.

An object of the present invention is to provide a very effective laser device for the selective treatment of acne.

An additional objective is to provide a laser device for selective acne treatment with reduced skin temperature rise, which prevents damage to surrounding tissue.

An additional object is to provide a laser device for selective acne treatment with reduced skin temperature rise capable of reducing the heating effects of the skin area involved in treatment.

According to the present invention, the aforementioned and other objects are achieved by a laser device for selective treatment of acne and by a method according to the appended claims.

Additional characteristics of the present invention are described in the dependent claims.

According to the present invention, a solution for the selective treatment of acne is provided, ensuring the optimization of the physical parameters that determine the temperature rise, ΔT, which creates thermal damage to the sebaceous gland, while simultaneously providing dynamic control of some of them:

- a wavelength, λ, equal to 1726 nm, generally in the wavelength range 1690 nm-1750 nm, whereby the heating effects due to absorption of water in the tissues surrounding the sebaceous gland are reduced, thus guaranteeing a minimum scattering effect and thus a maximum value fraction f;

- power P> 1 W, whereby a suitable energy density is guaranteed for a process that is very stable in radiation (fluctuations <3%) so that the depth of the process does not change for a long time;

- a laser beam with a flat-topped intensity distribution (η≤15%), i.e. suitable for the selective treatment of the sebaceous gland, which does not cause damage to the surrounding tissues and has a diameter φ> 0.5 mm, generally selected from the range of 0.5 mm-5 mm, keeping the distribution of the laser beam intensity unchanged and guaranteeing the possibility of penetration into biological tissue by the selected way;

- control of the temperature of the skin surface, T _i ϵ [-10 ° C; + 10 ° C], without the use of any cryogenic gas that can create thermal shock to human skin;

- the duration τ of the laser pulse, which should not be longer than the time of heat dissipation by the sebaceous gland, and should be such as to prevent heating of the tissue surrounding the gland.

The systemic object of the present invention allows to obtain an optimal temperature distribution in biological tissue in order to achieve a selective treatment of acne with the use of minimum laser energy by reducing the effect of interaction of laser radiation with said tissue.

It should be added that the proposed solution, in addition to overcoming the limitations of the acne treatment process, makes it possible to obtain a "flat-topped" beam, the intensity distribution of which does not depend on the conditions for the release of laser radiation and on the power of the said laser radiation.

The characteristics and advantages of the present invention will be apparent from the following detailed description of practical embodiments thereof, illustrated by way of non-limiting example in the accompanying drawings, in which:

fig. 1 shows an increase in temperature ΔT caused by a laser beam with a wavelength of 1726 nm, having a uniform (flat-topped) intensity distribution on the right and a Gaussian distribution on the left, in biological tissue, with an energy density of 50 J / cm ² and a beam diameter of 3.5 mm from above and 1 , 5 mm from below, and this simulation considers the sebaceous gland located on the Y-axis with R = 0 cm and at a depth of 0.6 mm from the skin surface, with the X-axis showing the depth in cm and the Y-axis showing the beam size in cm ;

fig. 2 shows the profile of the temperature increase ΔТ along the vertical axis of the sebaceous gland (R = 0) created by a laser beam with an energy density of 50 J / cm ² having a Gaussian intensity profile, with a change in the diameter of the optical laser beam from the lower curve to the upper curve equal to 0 , 25, 0.5, 1, 2, 3, 3.5, 4, 5 mm, and the dermis is located between segments A and D, and the sebaceous gland is located between segments B and C;

fig. 3 shows the profile of the temperature increase ΔТ along the vertical axis of the sebaceous gland (R = 0), created by a laser beam with an energy density equal to 50 J / cm ² , having a uniform intensity profile with a change in the diameter of the optical laser beam from the lower curve to the upper curve equal to 0.25, 0.5, 1, 2, 3, 3.5, 4, 5 mm, and the dermis is between segments A and D, and the sebaceous gland is between segments B and C;

fig. 4 schematically shows a laser system for the selective treatment of acne according to the present invention;

fig. 5 shows the evolution of the intensity distribution of a laser beam with a wavelength of 1726 nm, with a change in the length L of the fiber, for a fiber having a core diameter of 200 μm and a core NA of 0.22;

fig. 6 shows the dependence of the penetration z of radiation into biological tissue on the diameter φ of the laser beam;

fig. 7 shows the dependence of the thermal path R _th on the pulse duration τ;

fig. 8 shows the change in the temperature of the sebaceous gland in the case of one single pulse having a duration of 400 ms;

fig. 9 shows the change in the temperature of the sebaceous gland in the case of a sequence of three pulses with a duration of 100 ms.

When laser radiation, or more generally light radiation, reaches biological tissue, the first effect encountered is absorption of photons by the tissue. At the same time, the phenomenon of scattering of photons can be observed, and in some cases - the phenomenon of reflection, which compete with absorption. Physically, these processes depend on the tissue absorption coefficient (μa), scattering coefficient (μs) and anisotropy coefficient (g) for scattering, and on the ratios of refractive indices (n) for reflection. Secondly, the light absorbed by biological tissue (hereinafter also called target or target tissue) is converted into thermal energy (ΔE), which can be propagated into the surrounding tissues. Therefore, the temperature rise can be written as follows: (for ΔТ :) ΔТ = ΔЕ / (ρ * Cp) (equation 1), where ρ and Cp are the density and specific heat of the tissue, respectively. This temperature rise occurs not only on the target tissue, but also in adjacent tissues. The temporal trend of heat dissipation is determined by the thermal relaxation time (tr). Thermal relaxation time is defined as the time interval required for a Gaussian temperature distribution with a width equal to the diameter of the target tissue to decrease its central value by 50%. To a good approximation, tr [ms] is directly proportional to the square of the target tissue diameter and inversely proportional to the heat dissipation constant k: tr ~ d ² / (n * K), where n depends on the target geometry. For example, a sebaceous gland with a length of 0.1 mm heats up significantly after 0.5 s. The energy absorbed by the target tissue and the energy density of the incident radiation are related by the equation: ΔE ≈ μa * f * F (equation 2), where f is the fraction of the decrease in the intensity of the incident radiation before reaching the target tissue. If the intensity or better the energy density (F), defined as (incident radiation energy) / incident spot area), of the light radiation is sufficient, then the temperature rise destroys the target tissue according to equation 2. It should be remembered that the energy density (F) of the incident radiation can be written in terms of the laser power P and the pulse duration t as: F = (power * pulse duration) / (spot area), in which case we can talk about thermal damage , and light is said to have carried out the treatment. Combining Equation 1 and Equation 2, we have:

ΔТ = (Tf-Ti) ≈ [(P * τ) * (f * μa)] / [ρ * (Cp) * (π * (φ / 2) 2)] (equation 3),

from which it can be concluded that the increase in temperature that creates thermal damage is proportional to:

a) the absorption coefficient, μa, and thus depends on the wavelength λ of the incident radiation;

b) the fraction of f that decreases as the scattering phenomenon increases, and thus, with respect to the previous aspect, depends on the wavelength of the incident radiation and correlates with the depth z to which the light radiation penetrates the biological tissue;

c) the energy E of the incident radiation and thus the power P of the radiating system, by means of the relation E = P * τ;

d) the time τ of irradiation with light, which, if greater than the thermal relaxation time tr, can cause more heat to spread outside the target tissue and thus an excessive temperature rise;

e) diameter φ and intensity distribution over the area of the laser beam;

f) the initial value Ti of the target tissue temperature.

In this aspect, it is pertinent to discuss the biological importance of temperature ranges (ΔT). In many human tissues, an increase in temperature that creates a temperature in the range of 50 ° C-60 ° C is sufficient to create thermal damage, but at higher values, very undesirable effects occur. Specifically, in the temperature range of 60 ° C-70 ° C, protein structures and collagen are denatured, while in the temperature range of 70 ° C-80 ° C, nucleic acids are disaggregated and membranes become permeable. When the temperature reaches 100 ° C, the water contained in the tissues evaporates. It can be concluded that the method of creating thermal damage due to an increase in temperature (ΔT) in the target tissue by means of light radiation is of significant interest for aesthetic and medical applications, but, in the mentioned process, unwanted increases in temperature in the surrounding tissues, which can create side effects similar to those described above. It follows that in the aforementioned applications, it is most important to control during the process all the physical parameters on which the temperature rise depends (Equation 3). Of all the known studies, the most comprehensive are undoubtedly the publications of prof. RR Anderson. Selective photothermolysis (Anderson and Parrish, Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation in Science 220: 524-527 1983) is based on the principle that a suitable and maximum temperature rise (ΔT) occurs only in the selected target tissue, i.e. .e. damage caused by light or laser radiation is limited (Alora and Anderson, Recent Developments in Cutaneous Lasers in Lasers in Surgery and Medicine 26: 108-118 2000). Selective photothermolysis technology has been applied in a variety of fields, including the selective treatment of acne. US6605080 illustrates a method and apparatus for selectively targeting lipid-rich tissues and excellently reveals energy density values for selective treatment of acne, but leaves open the issue of penetration of incident radiation into biological tissue, assuming the same wavelength, given that, as again stated in US6605080, the sebaceous glands are located at a depth that is significant and ranges from 1 mm to 4 mm from the skin surface. In this regard, the applicant has observed that the penetration z of radiation into the skin depends on the energy density and, in particular, on the area, i.e. diameter φ of the laser spot (Fig. 1). US6605080 indicates energy density ranges and time ranges within which acne treatment should be performed. These ranges of values do not take into account the correlation indicated mathematically in Equation 3, existing with the efficiency of penetration into biological tissue. Again, US6605080 proposes multiple wavelength ranges (880 nm-935 nm, 1150 nm-1230 nm, 1690 nm-1750 nm and 2280 nm-2350 nm) in which acne can be selectively treated. According to selective photothermolysis, the best condition for the selective treatment of acne is provided when the lipid absorption coefficient (μ _alip ), which the sebaceous gland is rich in, is greater than the water absorption coefficient (μ _aH20 ), which is rich in the epidermis and the dermis, which is the surrounding tissue. the mentioned sebaceous gland. Thus, it is possible to obtain strong (selective) absorption in the sebaceous gland and not to obtain strong absorption in the surrounding water-rich tissues. The above condition is met in all wavelength ranges specified in US Pat. No. 6,605,080, but the above wavelength ranges are not equivalent for the task of treating acne on human skin for two reasons:

1 - when passing from the range of 880 nm-935 nm to the range of 1690 nm-1750 nm, the contribution of radiation to the power P, which reaches the surface of human skin, is 10 times smaller;

2 - the scattering effect decreases as the wavelength increases, and thus the fraction f changes;

3 - on the other hand, the penetrating power of light radiation increases with increasing wavelength.

Therefore, it is advisable to determine one single wavelength range or better the wavelength that the light source or better laser source should emit and optimize all parameters for selective acne treatment for this wavelength. In 2006 prof. Rox R. Anderson (Anderson et al., Selective Photothermolysis of Lipid-Rich Tissues: A Free Electron Laser Study Lasers in Surgery and Medicine 38: 913-919 2006) conducted preliminary trials of a 1720 nm free electron laser and came to the conclusion that the lipid selective absorption band at 1720 nm may be of interest for the selective treatment of surface targets (ie, with a maximum depth of 2 mm in the skin) such as superficial sebaceous glands. Later, in 2011, a fiber optic source based on Raman scattering was developed, capable of emitting laser radiation at a wavelength of 1708 nm (Alexander et al., Photothermolysis of sebaceous glands in human skin ex vivo with a 1.708 micron Raman fiber laser and contact cooling in Lasers in Surgery and Medicine 43: 470-480 2011). The decision to use a fiber-optic source based on Raman scattering, which thus guarantees operation in the best wavelength range for selective acne treatment, means that the resulting beam has a Gaussian intensity distribution. The limitation of this solution is the use of the aforementioned laser beam having an intensity distribution with a Gaussian profile. In fact, said bunch is not the most suitable for selective acne treatment. As pointed out by the authors, this profile can create damage to tissues located outside the sebaceous gland. In WO2011 / 084863A2, the same authors propose to use a laser beam having a more uniform spatial distribution than the spatial distribution typical of a Gaussian laser beam, but give no practical guidance on how to obtain it and how to make it effective for selective acne treatment. To reduce the damage caused by the excessive rise in skin surface temperature, a cooling system has been introduced into existing acne treatment devices. There are multiple solutions that determine the cooling system of the skin surface, i.e. they determine a suitable starting temperature Ti. Many of these solutions are based on the release of cryogenic fluids to the skin surface (Paithankar et al., Acne treatment with a 1,450 wavelength laser and cryogen spray cooling, Lasers in Surgery and Medicine 31: 106-114 2002). These solutions are often very complex and suboptimal when, during treatment, the value of the radiation energy density that creates the temperature rise, ΔT, must be greatly modified.

In conclusion, the prior art has different approaches for selectively treating acne, but there is no single general solution that would control and dynamically modify all the parameters that affect the temperature rise and which are mathematically expressed in Equation 3. Therefore, none of the solutions presented do not exclude the possibility of creating biological damage to the tissues surrounding the sebaceous gland.

Numerical Monte Carlo models were used to identify possible solutions to overcome the limitations of the prior art described above. In these models, the target tissue is represented by a sebaceous gland located in the skin, in particular in the dermis. The sebaceous gland is located, for example, at a depth of 0.6 mm from the skin surface and has a length equal to 1.0 mm. FIG. 1 shows Monte Carlo models which on the left show the temperature rise (ΔT) produced by a beam having a Gaussian intensity profile as the laser spot diameter decreases, and on the right show the temperature rise (ΔT) produced by a laser beam having a uniform intensity profile. (also called "flat top") when the diameter of the laser spot decreases. This discussion states that the intensity distribution of the laser beam is flat-topped; homogeneous when the ratio (η) between the standard deviation (δI) of the intensity and the mean (<I>) of the same intensity is less than the predetermined value here set at 15%. In the case of the same energy density, it can be observed that a beam having a uniform (flat-topped) intensity distribution creates a uniform temperature rise (ΔТ) in the first layers of the tissue, i.e. in the layers preceding the sebaceous gland. On the other hand, a laser beam having a Gaussian intensity profile creates a strong temperature rise (ΔT) gradient, especially in the first layers of tissue. This can be seen particularly clearly in FIG. 2. FIG. 2 shows the temperature rise (ΔT) profile along the vertical axis of the sebaceous gland (R = 0) produced by a laser beam having a Gaussian intensity profile (top left) and produced by a laser beam having a uniform intensity profile (bottom right) when the laser beam diameter changes. In the case of a laser beam with a Gaussian intensity profile with a diameter> 1 mm, a temperature rise of> 70 ° C is created in the skin layers preceding the sebaceous gland. Said increase is undesirable for said layer of skin. This effect does not occur in the case of a laser beam having the same energy density as the energy density of the previous laser beam, but with a uniform intensity distribution. In addition, in the case of a laser beam having a uniform (flat-topped) intensity distribution, when the beam diameter changes, the temperature rise dispersion (ΔT) decreases significantly. It can be concluded that a laser beam having a uniform intensity distribution is preferable to a laser beam having an intensity distribution with a Gaussian profile for the task of selectively treating acne without the side effect of damage to surrounding tissues. Analysis of the models by the Monte Carlo method indicates that as the diameter of the incident laser radiation increases, the penetration rate of z radiation into biological tissue increases. The advantage of modulating the spot diameter φ is thus obvious, and consists in maintaining a constant energy density of the process, in order to reach more or less deep layers of the skin. The use of a "flat-top" beam is preferred in various applications (EP2407807, US5658275), and there are multiple technologies for obtaining such a beam profile, starting with the intensity distribution of a multimode source. In particular, in US6532244, a "flat top" beam is obtained by injecting a multimode laser beam (V-number> 2.405) into two multimode fibers, the first fiber having a V-number lower than the second fiber; the second fiber, called the remote control fiber, is bent with a suitable radius of curvature (known as bending technology). Also known are solutions (WO2011070306) in which a laser beam having a Gaussian intensity profile is converted by nonlinear materials into a beam having a certain intensity distribution. A laser beam having an arbitrary intensity profile can be made flat-top also by means of special diffractive optics. The solutions mentioned are not particularly optimal. Specifically, an application in which a radius of curvature is provided on the fiber to produce a beam with a uniform intensity distribution is impractical due to problems associated with power loss introduced by curvature (D. Marcuse, "Curvature loss formula for optical fibers", J. Opt. Soc. Am 66 (3), 216 (1976)), and with the likelihood of creating microcracks in the fibers subjected to bending. The solution of switching from a fiber with a V-number V1 to a fiber having a V-number V2, where V2> V1, requires the use of optics that have the effect of breaking the wavefront and introducing a loss in light intensity. Finally, solutions that entail the use of discrete optics such as microlenses or nonlinear materials cause significant power losses when laser radiation passes through nonlinear materials. Finally, it is also known that in order to obtain a beam with a uniform intensity distribution, a non-uniform beam must be injected into the fiber at appropriate angles (Shealy and Hoffnagle Laser beam shaping profiles and propagation in Appl. Optics Vol 45 2006).

The laser apparatus for selective treatment of acne according to the present invention comprises a laser source 1 in an optical fiber based on a Raman effect. Source 1 ends in optical collimator 2. Collimator 2 is optically aligned with optical fiber 5 by means of an optomechanical interface 3. Optomechanical interface 3 consists of a linear and angular micrometric adjustment system (xyz,-ϕ), which, by means of a lens 4 located inside it, focuses collimated beam exiting from collimator 2 into the fiber core 5. Optomechanical interface 3 ends at SMA connector 6, and multimode fiber 5 begins at SMA connector 7.

Fiber 5 terminates in an SMA connector 8, which is connected to a tip 10, which is placed in contact with biological tissue during treatment, through an SMA connector 9, which interacts with the SMA connector 8.

The handpiece 10 contains a zoom optical system 11 that makes it possible to magnify the laser beam exiting from the fiber 5.

The tip 10 has a sapphire window 12 at its end.

The laser source 1 contains a switch 13 that interrupts the passage of the laser beam and makes it possible to adjust the duration of the sent laser pulses.

By activating the switch 13 accordingly, it is possible to send laser pulses having the required duration and separated by the required waiting times.

Source 1 emits at 1726 nm, or more generally in the 1720 nm-1730 nm range. The absorption coefficient of lipids is greater than the absorption coefficient of water, μ _alip = 10cm ^-1 > μ _aH20 = 6cm ^-1 (@ 1720nm), not only in the mentioned range, but the scattering coefficient (3.5cm ^-1 @ 1720nm) decreases significantly relative the absorption coefficient of lipids (10cm ^-1 @ 1720nm), which guarantees the fulfillment of the condition that almost all incident photons are absorbed by biological tissue. The radiation emerging from the optical collimator 2 is collimated and has a diameter in the range of 3 mm-5 mm. Source 1 can emit laser radiation in a continuous mode or in a pulsed mode. The laser source 1 is equipped with a power regulator and a switch that provides pulsed radiation of the source. Taking into account the nature of the source 1, the intensity profile of the radiation emerging from the collimator 2 has a Gaussian shape. In an alternative configuration, the laser source may terminate in fiber having a V-number> 2.405.

The optomechanical interface 3 consists of a linear and angular micrometric adjustment system (x-y-z, ϴ-ϕ), which by means of a lens 4 focuses the collimated beam emerging from the collimator 2 into the fiber core 5.

Fiber 5 has the following characteristics:

1. The diameter φ and the numerical aperture NA of its core are non-functional to create a beam having a uniform intensity distribution, but are functional to ensure that the collimated laser beam input through lens 4 is maximized so that no loss of light intensity and unwanted overheating of the SMA are introduced. -connector 7;

2. V-number> 2.405;

3. The core can be round, square or rectangular;

4. Length L, to obtain, after a certain value L, called L *, a laser beam having a uniform intensity distribution;

5. It is coiled with a radius of curvature that is functional only for placement in the device and does not introduce radiation intensity loss due to bending.

FIG. 5 shows the intensity distribution of the laser beam emerging from the fiber 5 for different values of the fiber length L. As an example, for a fiber having a V-number = 78.50, the L value for which the intensity distribution is uniform (η≤15%) is an L * value greater than or equal to 25 m.If we consider η≤20% , then the length L * will be greater than or equal to 15 m.

It should be noted that when the length L of the fiber satisfies the inequality L ≥ L *, the parameter η is independent of the release conditions, for example, the specifications of the lens 4. Thus, achieving this latter result makes one of the physical parameters η functional for selective acne treatment, regardless of what are the conditions for optical alignment of the system, which may change over time. In addition, the technical choice of using only the fiber length parameter as a control element for creating a flat-topped beam has the advantage that it does not introduce any power loss P of the laser source 1. In conclusion: the technological solution chosen to obtain suitable uniformity (η ≤15%) of the intensity distribution for the selective treatment of acne, is independent of the laser power P required for the treatment.

It was found that L * depends on the V-number of the fiber and the wavelength of the incident laser radiation. In particular, it was found that L * decreases when the V-number increases and L * increases when the wavelength decreases. In conclusion, not only is the selected 1720 nm-1730 nm wavelength range preferable for the coefficient values described above, but the resulting intensity uniformity value is less. In the proposed solution, the radius of curvature with which the fiber is placed in the device has no effect on the transformation of the intensity distribution into a uniform intensity distribution. Finally, the fiber 5 that satisfies the 5 above conditions is an element that converts a laser beam having an intensity distribution with a Gaussian intensity profile exiting from the collimator 2 into a laser beam having a uniform intensity distribution. Similarly, the fiber 5 can convert a non-uniform laser beam with a non-uniform intensity distribution into a beam having a uniform intensity distribution.

The fact that fiber 5 and ferrule 10 are connected by two SMA connectors makes ferrule 10 a removable element, i. E. this is very useful in a given application in case of malfunction or damage during treatment.

The zoom system 11 consists of an optical system configured to create on the sapphire window 12, which is located in the image plane of the said system, an enlarged image of the output surface of the fiber 5, which guarantees the same intensity distribution.

The zoom system 11 is an optical system of 3 lenses. As an example, the first lens 11a is a plano-convex lens that focuses the beam exiting from the fiber 5 onto the second lens 11b. The second lens 11b is a biconcave lens. The third lens 11c is a biconvex lens that converts the magnified beam coming from the second lens 11b into a collimated beam that reaches the window 12. The second biconvex lens 11b, moving between the first lens 11a and the third lens 11c, scatters the light beams, thereby modifying increase of the beam emerging from the fiber 5.

The movement of the second lens 11b takes place in a known manner and can be continuously adjusted from the outside.

In an alternative solution, after the third lens 11c, an additional fourth weakly convex lens 11d can be introduced, which makes it possible to focus the enlarged beam inside the biological tissue. The magnification mʺ provided by the zoom system 11 is variable, so that the most suitable laser beam diameter φ is dynamically provided during treatment. This optical configuration does not change the intensity distribution of the laser beam.

As an example, suppose fiber 5 is a fiber having a core diameter of 0.2 mm, the zoom system 11 dynamically achieves a magnification of 2.5x to 25x so that it produces a laser beam diameter in the sapphire window ranging from 0.5 mm to 5.0 mm, and more preferably 1.5 to 3.5 mm. This solution has the unique characteristic of modifying two process parameters during treatment: energy density and thus an increase in temperature ΔT in the target tissue, and the size of the output laser beam, and thus the level of depth in the tissue achieved by radiation ( Fig. 6 and 7). It should be emphasized that the mentioned dynamism does not affect the level of uniformity of the intensity distribution of the laser beam. In addition, it is possible to introduce a feedback system that links the magnification produced by the system 11, and thus the spot diameter φ, with the adjustment of the power P emitted by the laser source 1 so that for each spot diameter that is provided on the skin surface , provide a suitable energy density. As an example, if we want to create an energy density of 50 J / cm ² using a beam having a uniform intensity distribution and a diameter of 3.5 mm, then a laser power of approximately 60 W may be required. If during the same treatment it is required to reduce the energy density, for example, from 50 J / cm ² to 30 J / cm ² , without changing the depth of the process, i.e. while maintaining a spot with a diameter of 3.5 mm, it will be sufficient to reduce the power of source 1 to approximately 36 W. Additional example: if we want to apply an energy density of 30 J / cm ² in the case of a beam having a uniform intensity distribution and a diameter of 4.0 mm, then a laser power of approximately 62 W may be required. If during the same treatment it is necessary to reduce the depth of the process without changing the energy density, then it will be sufficient to reduce the spot size to 2.0 mm and the power of the laser source 2 to 19 W. FIG. 6 shows the dependence of the depth z of the process on the size φ of the beam. It should be noted that during acne treatment it is not problematic to reach the sebaceous glands located in the more superficial layers of the skin, but it is more difficult to reach the sebaceous glands located deeper. The proposed solution solves this critical problem, as it dynamically provides the same treatment for both superficial sebaceous glands and deeper sebaceous glands, or, in general, sebaceous glands located at a depth of 0.5 mm to 5.0 mm ... An additional advantage of the proposed solution will be apparent if we consider the positions of pain receptors in the skin. They are located in the surface areas of the skin at a depth of z <2.5 mm and have an average density of approximately 100 / cm ² . It follows that for the treatment of sebaceous glands located near the skin surface, for example, in the range z диапазоне [0.5 mm; 2.5 mm], and reducing the stimulation of the greatest number of receptors, it is advisable to work with beams with diameters φ <2.0 mm.

In some cases, to reduce damage to the surface layers of the skin, it is advisable to reduce the temperature of these layers. To reduce the temperature of the first layers of the skin, a cooling system (not shown) connected to the tip 10 can be used, which, by means of an air stream emitted from the tube 14, can reduce the temperature of the sapphire window 12 located after the zoom system 11; the sapphire window 12 is placed in contact with the biological tissue to be treated. The mentioned cooling system allows you to adjust the temperature in the range from -10 ° С to + 10 ° С. This solution has a double advantage: on the one hand, the water currents are not used to cool the window 12, but, on the other hand, the air flow that hits the inner side of the window 12, i.e. the side opposite to the contact surface with the biological tissue to be treated prevents the formation of condensation formed at low temperatures to which the element 12 is exposed. The sapphire window 12 is, in general, an optical window selected for its high thermal conductivity value and transparency for radiation of interest, but does not change the shape of the intensity profile of the laser beam.

The cooling process of the various layers of the skin below the surface of the skin is regulated by the laws of thermodynamics. With this in mind and the presence of a switch 13 of the laser source 1, the duration of the pulse or pulse train to be applied can be adjusted as shown in FIG. 8 and 9.

At the moment t = 0 ms, the sebaceous gland is irradiated with power P for time τ. The sebaceous gland temperature rises from the basal tissue temperature T _base to the maximum temperature T _peak , i.e. an increase in temperature ΔТ is provided. The duration τ of the laser pulse is shorter than the thermal relaxation time of the target tissue (in this example, it is 450 ms), which, as discussed above, depends on the geometry of the target so as not to cause heating of the surrounding tissue. At the end of the irradiation, the temperature decreases, and after a while the temperature of the sebaceous gland returns to T _base .

If the duration τ of the pulse, i.e. the time interval during which the power P is provided is insufficient to create an increase in temperature ΔT, which creates thermal damage in the sebaceous gland, then the pulse duration τ is increased until the maximum limit represented by the thermal relaxation time is reached. This results in heating of the tissue areas surrounding the sebaceous gland due to the release of energy absorbed by the sebaceous gland. The size of these areas depends on the pulse duration and on the thermal path Rth (FIG. 7), which represents the radial propagation of the energy released by the irradiated sebaceous gland, and depends on the duration of the irradiation.

The proposed solution avoids heating of the tissue areas surrounding the sebaceous gland by temporarily modulating the laser pulse. FIG. 9 shows an example of said modulation for a sebaceous gland with a thermal denaturation temperature of 50 ° C and tr ~ 500 ms. FIG. 8 shows the value of the temperature generated by a source that emits a 400 ms pulse. FIG. 9 shows the value of the temperature generated by the same source, which emits three pulses, each with a duration of 100 ms, separated by a time interval of 500 ms, according to the present invention. In the first case, the radial spread is 0.45 mm, in the second case, according to the present invention, said value is reduced by 50%, i.e. it reached a value of 0.22 mm. By way of example, the source 1 pulse can be modulated in the range between 10 ms and 500 ms.

If single laser sources with a power suitable for performing selective acne treatment are not available, an alternative solution is provided in which two or more sources are combined.

Claims (9)

1. A laser device for the selective treatment of acne, containing a laser source (1), ending in an optical collimator (2), which supplies a laser beam; moreover, said laser source (1) comprises a switch (13) that allows the transmission of pulses of said laser beam of a given duration; an optomechanical interface (3) containing a lens (4) focusing a laser beam received from an optical collimator (2), an optical fiber (5) connected to said optomechanical interface (3), characterized in that said optical fiber (5) has length greater than 15 m, and said device contains a tip (10) connected to said optical fiber (5), and said tip (10) contains an optical zoom system (11), which makes it possible to change the diameter of the laser beam emerging from said tip ( 10), from 0.5 to 5 mm. 2. A device according to claim 1, characterized in that said laser source (1) emits a laser beam at a wavelength of 1726 nm. 3. A device according to one of the preceding claims, characterized in that said laser source (1) is single-mode. 4. An apparatus according to one of the preceding claims, characterized in that said optical fiber (5) is a multimode fiber. 5. The device according to one of the preceding claims, characterized in that said optical fiber (5) generates at its output a laser beam having a flat-topped beam intensity distribution, the ratio between the standard deviation of the intensity and the mean value of the same intensity being less than 20%. 6. An apparatus according to one of the preceding claims, characterized in that said tip (10) comprises at its outlet a sapphire window (12). 7. A device according to one of the preceding claims, characterized in that it comprises a cooling system that directs air at a predetermined temperature to the inner surface of said sapphire window (12). 8. An apparatus according to one of the preceding claims, characterized in that said switch (13) is controlled such that the pulse duration is between 10 ms and 550 ms. 9. A method for selective treatment of acne using the device according to claim 1, comprising the steps of applying pulses of a laser beam at a wavelength of 1726 nm to a multimode optical fiber (5) having a length equal to or greater than 15 m, supplying a laser beam emerging from the said optical fiber (5), to the tip (10), the optical zoom system (11) located inside the said tip (10) is adjusted to obtain a given diameter of the laser beam emerging from the said tip (10), ranging from 0.5 to 5 mm.

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